This invention relates to a photochromic plastic object made of a transparent plastic (i.e., synthetic resin) material composed of at least two interpenetrating polymer networks of different polymer materials and containing at least one photochromic dye homogenously distributed therein. The invention also relates to a method for the production thereof.
A number of methods for the production of photochromic plastic objects, especially lenses for eyeglasses, are known in the art and are used industrially: namely mass dying, i.e., adding the photochromic dyes to the casting resin mixture prior to polymerization; surface dying, where the photochromic dyes are introduced by thermal diffusion into at least one surface of the at least partially cured plastic material forming the plastic object (“matrix”) typically using an intermediate carrier that absorbs the photochromic molecules prior to diffusion and that is subsequently removed again; and coating, where an approximately 10-50 μm thick photochromic film is applied to the cured plastic object.
In all cases the surrounding matrix strongly influences the photochromic properties of the dyes used. To a lesser degree this is true for the color, to a greater degree for the service life, i.e., the period for which the corresponding photochromic plastic object can be used, and in particular for the kinetic properties, i.e., the darkening and brightening rate, the darkening depth and their dependence on the temperature.
Thus, synthetic resin materials for use with photochromic dyes should meet two—usually contradictory—requirements. On the one hand they should offer sufficient free space so that after excitation by light the photochromic molecules can structurally rearrange themselves as freely as possible into the open, colored form and at the end of exposure to light or with the onset of darkening can revert back to their closed, colorless form. For most fields of application, this requires a narrow time window. For darkening this time window typically ranges from 10 to 100 seconds and for brightening from 1 to 10 minutes. On the other hand, since most photochromic molecules in their open form are sensitive to atmospheric oxygen, the synthetic resin matrix must be as resistant to diffusion as possible. This diffusion resistance is difficult to reconcile with a polymer network with large free spaces. The currently available plastic or synthetic resin materials have either a good service life—associated, however, with comparatively slow darkening and brightening kinetics—or they exhibit rapid reaction kinetics in response to different lighting intensities—but only within an extremely limited lifetime. The first group includes synthetic resin materials that are used in comparatively expensive consumer articles, e.g., lenses for eyeglasses, which are expected and required to have a service life of several years. Characteristic for this group, as a rule, is the use of a typically rigid synthetic resin matrix with a material thickness of at least 1 to 2 mm. The second group includes synthetic resin materials primarily used for novelty or fashion items, which are not expected to have a long service life, e.g., toys or consumer textiles, such as T-shirts. The synthetic resin matrix used for this purpose is usually flexible, e.g., in the form of films, or is not dimensionally stable in itself, e.g., in the case of coatings and varnishes. Here, the material thickness of the photochromic synthetic resin matrix is typically less than 1 millimeter.
Synthetic resin materials whose polymer network is formed by a chemical reaction such as polyaddition or polycondensation are currently used exclusively in the prior art. This formation can also be effected in steps: i.e., by using different initiators, a preliminary cross-linking is effected first and a final cross-linking at an elevated temperature thereafter. Known, for example, are acrylates, especially for eyeglass lenses, epoxy resins (EP-A-0 556 646, JP-A-47 015 122, DE-A-33 08 186) or polyurethanes and polythiourethanes (U.S. Pat. No. 4,889,413). For contact lenses polyureas are also used (EP-A-0 735 097, EP-A-0 748 685). FR 2,674,529 discloses a photochromic synthetic resin object based on a polyallyl carbonate of bisphenol A, where a photochromic dye is introduced by diffusion into the synthetic resin object after its polymerization. This dyeing technique involves allylic monomers requiring very aggressive initiators (PIP) in high concentrations (approximately 3%). However, introducing a photochromic dye by diffusion in a dye bath into the plastic object usually reduces the photochromism and thus results in an unsatisfactory service life of the photochromic synthetic resin object.